1. Field of the Invention
The invention relates to methods for strengthening bonds, and more particularly, to methods for strengthening bonds between thin-film conductors and insulating substrates on which they are formed and, thereby, connection of external leads made to the conductors. 2. Description of the Prior Art
Films are divided, in the electronic art, into two general classes: thick films and thin films. The thick films originate from a paste, of metallic particles and a vehicle, which is silk screened onto a substrate. Such films are usually fired (see Paul Sayers, "Molecular Bonding Conductive Films," Solid State Technology, September 1974, pp. 66-69) at 800.degree. C and higher. Typically, the films are 250,000A (angstroms) (0.001 inches) or more thick but may be as little as 50,000A (0.0002 inches) thick after firing. Thin films, on the other hand, originate from direct vacuum deposition of metals on a substrate and may be 125,000A (0.0005 inches) thick but typically are less than 30,000A thick. This is in accord with Berry, Hall and Harris, Thin Film Technology, D. VanNostrand, Princeton, New Jersey (1968), pg. 2, where it is stated that "Thin films for electronic circuit applications range from a few hundred angstroms to tens of thousands of angstroms in thickness."
In order to make thin-film resistors, a film of tantalum or other resistive material is deposited on a substrate, usually by sputtering. A conductive material, which may be deposited either or both by evaporation and plating is also required in order to form contact pads and/or conductors that will permit electrical connections to be made to the resistors. The electrical connections are generally made by thermocompressively bonding external leads to the contact pads and/or conductors and it is desirable to improve the strength of these bonds.
In the past, it has been recognized that thin film resistors were stabilized by heat treatment (see U.S. Pat. No. 3,159,556 issued to D. A. McLean et al. on Dec. 1, 1964) but the workers in the thin film art did not recognize that a correlation existed between heat treatment and the strength of the external lead bonds subsequently made to the contact pads or conductors.